1. Field of Invention
The present invention relates to a sensor device with dark current compensation and a control method thereof; particularly, it relates to such sensor device and control method thereof which generate an analog compensated signal with dark current compensation according to an analog sensing signal.
2. Description of Related Art
FIG. 1A shows a schematic diagram of a typical sensor device 100. As shown in FIG. 1A, the sensor device 100 includes: a sensor circuit 11, an analog gain circuit 13, an analog-to-digital converter (ADC) circuit 15, and a digital processing circuit 18. The sensor circuit 11 senses a physical property or a chemical property (sensed property hereinafter) of an object or an environment to generate an analog sensing signal. The analog gain circuit 13 receives the analog sensing signal, and generates an analog gain signal. The ADC circuit 15 receives the analog gain signal, and converts it to a digital gain signal. The digital processing circuit 18 receives the digital gain signal, and removes a power noise and a dark current therein to generate a digital sensing signal. As shown in FIG. 1B, the analog sensing signal includes a power noise signal PN, a dark current signal DC, and a target signal SIG.
The sensor device 100 for example is an image sensor device, and the sensor circuit 11 for example is an image sensor circuit. The image sensor circuit senses an image to generate the analog sensing signal. The analog gain circuit 13 amplifies the analog sensing signal to generate the analog gain signal, wherein the analog gain signal includes a power noise signal PN1, a dark current signal DC1, and a target signal SIG1 as shown in FIG. 1B, wherein the target signal SIG is related to the sensed image. The power noise signal PN1, the dark current signal DC1, and the target signal SIG1 are obtained by amplifying the power noise signal PN, the dark current signal DC, and the target signal SIG respectively. The ADC circuit 15 converts the analog gain signal to the digital gain signal. The digital processing circuit 18 removes a power noise signal PN2 and a dark current signal DC2 in the digital gain signal to generate the digital sensing signal. However, referring to FIG. 1B, the information in the digital sensing signal may be lost during the above process. This is because the analog sensing signal which includes the power noise signal PN, the dark current signal DC and the target signal SIG is amplified by the analog gain circuit 13, and the generated analog gain signal may exceed an overflow level as shown in the figure. Information in the portion of the analog gain signal above the overflow level is discarded. For example, when the sensor device 100 processes in a high temperature environment, the dark current doubles for every 6-7 degrees increase of the ambient temperature. For a more specific example, a car electronic equipment with the sensor device 100 may operate at an ambient temperature between 80 to 100 degrees Celsius. As compared with operating at a room temperature of 25 degrees Celsius, the dark current generated by the sensor device 100 may increase dozens of times or more. Therefore, the analog gain signal which is obtained by amplifying the analog sensing signal may exceed the overflow level because the dark current signal is too high. Excess portion of the analog gain signal over the overflow level is discarded. Referring to FIG. 1B, the target signal SIG1 and part of the dark current signal DC1 of the analog gain signal are discarded because of overflow, so the ADC circuit 15 converts the remaining part of the analog gain signal with only the power noise signal PN1 and the remaining part of the dark current signal DC1 to the digital sensing signal which includes only the dark current signal DC2 and the power noise signal PN2. The digital processing circuit 18 removes the dark current signal DC2 and the power noise signal PN2 to generate the digital sensing signal, but the digital sensing signal is null because the digital gain signal loses the information of the target signal SIG1 which is discarded because of overflow. Note that the signal bar chart is for illustrative purpose, not for limiting the distribution of the power noise signal, the dark current signal, and the target signal. For example, the target signal SIG1 does not have to be on top of the analog gain signal bar chart. No matter where the target signal SIG1 locates in the analog gain signal bar chart, it will be lost in the digital sensing signal if overflow occurs.
The power noise signal PN is generated from a noise of a power source, which is well known by those skilled in the art, so details thereof are omitted here. The target signal SIG may be lost in the prior art sensor device 100 when it, for example, processes in a high temperature environment, which causes the dark current signal of the analog gain signal to increase dramatically. In this case, no image will be taken and outputted by the image sensor device.
In view of above, to overcome the drawbacks in the prior art, the present invention proposes a sensor device with dark current compensation and a control method thereof, which can retain the target signal under a high dark current condition, such that the target signal is not lost when the analog sensing signal is converted to the digital sensing signal.